Devices and methods for hydrogen generation via ammonia decomposition

ABSTRACT

Systems and methods for hydrogen generation via ammonia decomposition that utilize a fixed bed reactor configured to receive inflows of NH 3  and oxidant and to produce an outflow of high purity H 2 . The fixed bed reactor contains a fixed bed of a NH 3  decomposition catalyst wherewith the NH 3  decomposes to form N 2  and H 2 ; a plurality of ceramic hollows fibers with a high surface to volume ratio disposed in the fixed bed, the hollow fibers having an H 2  selective membrane disposed thereon for extracting H 2  from N 2  and to form a permeate of the high purity H 2  and a retentate of primarily N 2 ; and a catalytic H 2  burner also disposed in the fixed bed, the catalytic H 2  burner for burning a portion of the H 2  with the oxidant to provide thermal energy for the NH 3  decomposition.

CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional PatentApplication, Ser. No. 62/541,962, filed on 7 Aug. 2017. The ProvisionalApplication is hereby incorporated by reference herein in its entiretyand is made a part hereof, including but not limited to those portionswhich specifically appear hereinafter.

GOVERNMENT RIGHTS

This invention was made with government support under Contract No.DE-AR0000931 awarded by the U.S. Department of Energy. The governmenthas certain rights in the invention.

BACKGROUND OF THE INVENTION Field of the Invention

This invention relates generally to hydrogen generation and, moreparticularly, to hydrogen generation via ammonia decomposition includingdevices, systems and methods particularly suited therefore.

Description of Related Art

There is a need and demand for hydrogen generation from a carbon neutralliquid fuel (CNLF) with high yield and sufficient purity for use incommercial proton exchange membrane (PEM) fuel cells. Required metrics,set by the US Department of Energy (DOE) Advanced Research ProjectsAgency-Energy (APRA-E), for such hydrogen generation include:

-   -   Conversion to hydrogen>99%    -   H₂ generation rate>0.15 g/h/cm³    -   Energy efficiency>80%    -   Maximum NH₃ decomposition reactor temperature of 450° C.    -   Hydrogen delivered cost at target pressure (30 bar)<$4.5/kg

It is very challenging for existing NH₃ cracking technologies to meetthese metrics due to a variety of facts and factors, including:

-   -   Low reaction temperature (<450° C.) and high NH₃ pressure (10˜15        bar) limit NH₃ conversion to less than 95%;    -   NH₃ decomposition is an endothermic reaction and thus requires        energy input; and    -   The cracked H₂ is mixed with N₂, which decreases the efficiency        of the PEM fuel cell. For example, with 60% dilution with N₂,        current density may decrease 8˜30% at 60° C.

Apollo Energy System Inc. (USA) designed an NH₃ cracking device togenerate H₂ for fuel cells. H₂-containing anode off gas was used as afuel for combustion to provide thermal energy for the cracker.Conversion to H₂ was high (99.99%) due to the high reactor temperature(480˜660° C.). The energy efficiency was not reported, but it is likelyhigh due to the efficient use of thermal energy from H₂ combustion forreactor heating. An efficient commercial catalyst (70 wt % Ni on Al₂O₃)modified with Ru was used. Therefore, a high H₂ generation rate was alsoobtained. Membrane reactors have been studied for H₂ generation from NH₃decomposition. However, the technology is still at the early researchstage, and both the Hz generation rate and conversion to H₂ are muchlower than ARPA-E's targets (see Table 1).

SUMMARY OF THE INVENTION

The present development is expected to solve at least some andpreferably each of the above-identified intrinsic issues and desirablysatisfy at least some and preferably each of the above-identifiedmetrics.

The present development can be adopted for providing high purity H₂ athigh rate and low cost from NH₃ decomposition for PEM fuel cellapplication as well as significantly reducing fuel transportation andstorage cost. As detailed below, the subject novel compact membranereactor also allows the use of NH₃ as an effective H₂ source for manyother potential applications that utilize H₂ fuel. This may lead toentirely new markets. Furthermore, via this platform, NH₃ can beeffectively coupled with maturing H₂ utilization technologies, allowingit becoming a new generation of fuel for wide applications.

Systems and methods in accordance with selected aspects of the subjectdevelopment can advantageously employ a membrane reactor containing: alow-cost, highly active catalyst for NH₃ decomposition, an H₂ selectivemembrane on ceramic hollow fibers with high surface to volume ratio forextracting Hz from N₂ (simultaneously shifting NH₃ decompositionreaction), and a catalytic H₂ burner to provide thermal energy for NH₃decomposition.

A system for generating hydrogen via ammonia decomposition in accordancewith specific embodiment of the subject invention development desirablyincludes a fixed bed reactor configured to receive inflows of NH₃ andoxidant and to produce an outflow comprising high purity H₂. The fixedbed reactor includes or contains a fixed bed of a NH₃ decompositioncatalyst wherewith the NH₃ decomposes to form N₂ and H₂; a plurality ofceramic hollows fibers with a high surface to volume ratio disposed inthe fixed bed, the hollow fibers having an H₂ selective membranedisposed thereon for extracting H₂ from N₂ and to form a permeatecomprising the high purity H₂ and a retentate comprising primarily N₂;and a catalytic H₂ burner also disposed in the fixed bed, the catalyticH₂ burner for burning a portion of the Hz with the oxidant to providethermal energy for the NH₃ decomposition.

A method for generating hydrogen via ammonia decomposition in accordancewith one aspect of the development involves introducing ammonia intosuch a membrane reactor so as to decompose at least a portion of theammonia to form ammonia decomposition products including hydrogen gasand then separating at least a portion of the hydrogen gas from thedecomposition products.

In one embodiment, a method for generating hydrogen via ammoniadecomposition is provided that involves introducing and decomposingammonia to form hydrogen and then separating and recovering high purityhydrogen via a specified fixed bed reactor system. More particularly,the fix bed reactor system includes a fixed bed membrane reactor thatcontains a fixed bed of a NH₃ decomposition catalyst for NH₃decomposition to form N₂ and H₂. The reactor also includes a pluralityof ceramic hollows fibers having a high surface to volume ratio disposedin the fixed bed. The hollow fibers have an H₂ selective membranedisposed thereon for extracting H₂ from N₂ and to form a permeatecomprising high purity H₂ and a retentate comprising primarily N₂. Thereactor further includes a catalytic H₂ burner also disposed in thefixed bed. The catalytic H₂ burner is adapted and effective for burninga portion of the H₂ to provide thermal energy for the NH₃ decomposition.Thus, the step of decomposing at least a portion of the ammonia isdesirably achieved via the fixed bed of the NH₃ decomposition catalystand thermal energy produced or provided by the catalytic H₂ burner toform ammonia decomposition products including H₂. The plurality ofceramic hollows fibers having a high surface to volume ratio disposed inthe fixed bed desirably serve to and are effective to separate andrecover high purity hydrogen from the decomposition products.

BRIEF DESCRIPTION OF THE DRAWINGS

Objects and features of this invention will be better understood fromthe following description taken in conjunction with the drawings,wherein:

FIG. 1 is a simplified schematic of a system for generating hydrogen viaammonia decomposition and more particularly a membrane reactor for H₂generation via thermal catalytic NH₃ decomposition in accordance withone aspect of the subject development.

FIG. 2 is a photomicrograph of designated portion A shown in FIG. 1,e.g., low-cost, highly active Ru-based catalyst for NH₃ decompositioncontained within the membrane reactor in accordance with one aspect ofthe subject development.

FIG. 3 is a simplified schematic representation of designated portion Bshown in FIG. 1, e.g., showing an H₂ selective membrane on ceramichollow fibers with high surface to volume ratio for extracting H₂ fromN₂ (simultaneously shifting NH₃ decomposition reaction) in accordancewith one aspect of the subject development.

FIG. 4 is a simplified schematic representation of designated portion Cshown in FIG. 1, e.g., a catalytic H₂ burner to provide thermal energyfor NH₃ decomposition in accordance with one aspect of the subjectdevelopment.

FIG. 5 is a simplified schematic of an H₂ production test system inaccordance with one aspect of the subject development and referenced inExample 1.

FIG. 6 is a simplified schematic of an HYSIS simulation systemreferenced in Example 2.

DETAILED DESCRIPTION

The subject development provides a novel compact intensive and modularNH₃ decomposition (2NH₃

N₂+3H₂) device with high conversion and high energy efficiency (>70%) atrelatively low temperature (<450° C.).

An exemplary system or device in accordance with the subject developmentis expected to generate high purity H₂ at high rate fromclose-to-complete NH₃ conversion, in a small reactor volume.

A system or device for generating hydrogen via ammonia decomposition,generally designated by the reference numeral 10 and in accord with oneembodiment of the subject development, is shown in FIG. 1. The system 10includes a membrane reactor 12 that includes or comprises three keycomponents: i) a fixed bed of low-cost, highly active catalyst for NH₃decomposition, generally designated by the reference numeral 14 and morespecifically shown in FIG. 2, ii) an H₂ selective membrane on ceramichollow fibers with high surface to volume ratio, generally designated bythe reference numeral 16, for extracting H₂ from N₂ and simultaneouslyshifting NH₃ decomposition reaction, more specifically shown in FIG. 3and a catalytic H₂ burner to provide thermal energy for NH₃decomposition, generally designated by the reference numeral 18 and morespecifically shown in FIG. 4.

In these figures, certain of the process streams are identified asfollows:

101 Feed—NH₃

102 Cracked stream—N₂+H₂ (−75%)

103 Product—H₂ (high purity)

104 Membrane retentate—N₂+H₂ (−25%)

105 Air—feed to the catalytic burner

106 Side product—N₂+H₂O

As shown in FIGS. 1-4, a feed stream of NH₃ 101 is introduced into themembrane reactor 12 such as through an inlet 22. In the reactor 12, theNH₃ contacts the NH₃ decomposition catalyst fixed bed 14 and undergoesdecomposition/cracking to form nitrogen (N₂) and hydrogen (H₂) gas, seeFIG. 2. The 142 selective membrane on ceramic hollow fibers with highsurface to volume ratio 16 serve to separate or extract H₂ from N₂ fromthe cracked stream, stream 102, see FIG. 3, and form a stream 103 ofhigh purity H₂ such as discharged or recovered from the reactor, such asvia the product outlet 24. Suitable ceramic hollow fibers includesaluminum hollow fibers but other ceramic materials such as known tothose in the art can be used, if desired. As shown, catalyst candesirably be loaded on the external surface of the hollow fibers 30. Asshown in FIG. 3, the ceramic hollow fibers 30 in a dense tube 31, e.g.,stainless steel tube, wherethrough the H₂ permeate is recovered, stream103, and the membrane retentate is passed on, stream 104. As shown inFIG. 3, active catalyst 32 can also be loaded in the ceramic hollowfibers 30 to assist in the decomposition residual NH₃.

As noted above, the reactor 12 includes and/or contains a catalytic H₂burner 18 such as to provide thermal energy for NH₃ decomposition. Asshown in FIG. 4, the catalytic H₂ burner 18 can desirably be formed ofor include a metal tube 40 such as containing a H₂ oxidizing catalyst42, such as known in the art. As shown in FIG. 1 and FIG. 4, thecatalytic H₂ burner 18 desirably serves to burn a portion of theproduced H₂ (for example, such as hydrogen in the membrane retentate,stream 104 reacting with via intake oxidant, e.g., air, such as stream105) to provide thermal energy for the NH₃ decomposition.

The subject development can and desirably does provide or result in thefollowing advantages/characteristics:

-   -   Pressurized NH₃ vapor at 10˜15 bar (stream 101) will be used        directly as feedstock;    -   Low-cost, highly active catalysts (for example, ruthenium-based        NH₃ decomposition catalysts such as known in the art or other        suitable NH₃ decomposition catalysts such as known in the art)        will be used in a compact fixed bed reactor for high rate NH₃        decomposition (2NH₃        N₂+3H₂) at reaction temperature below 450° C.;    -   H₂ selective membrane on ceramic hollow fibers with high surface        to volume ratio will be used as a reactor boundary to extract        high purity H₂ from the reaction product; the removal of the H₂        will also shift the reaction towards higher conversion;    -   If needed, active catalyst will also be loaded in the ceramic        hollow fiber to decompose residual NH₃;    -   Lower concentration residual H₂ in the retentate (stream 104)        will be burned with air in a catalytic burner to provide thermal        energy uniformly needed for NH₃ decomposition;    -   Permeated high purity H₂ (stream 103) and exhaust from catalytic        H₂ combustion (stream 106) at elevated temperature will be used        to preheat NH₃ feed (stream 101); and    -   High purity H₂ (>99%) (stream 103) after heat exchange will be        compressed for applications.

Table 1, below, shows a comparison of the invention with current andemerging technologies for H₂ generation from thermocatalytic NH₃decomposition and also with the ARPA-E technical targets.

TABLE 1 Comparison between technologies and ARPA-E technical performancetargets Bimodal catalytic Apollo membrane Palladium Invention ARPA-Energy reactor membrane membrane Description E target System^([1])(BCMR)^([3]) reactor^([4]) Reactor H₂ delivered cost at target <$4.5/kgN/A N/A N/A $4.078/kg pressure Final prototype size, L H₂/min 10 7.50.044 0.081 10 H₂ generation rate, g H₂/h/cm³ >0.15 0.126 0.038 0.01440.2 Conversion to H₂ >99% >99.99% 74%     85%  >99% Energyefficiency >80% N/A N/A N/A 88.45% Maximum cracking temperature, 450480~660 400~450 500~600 450 ° C. H₂ delivered pressure, bar 30 1 0.005 130 Life time (projected), yrs 10 N/A N/A N/A 10 Concentration ofcatalyst <100 ppb <100 ppm^(a) NH₃ (4.5%) <0.8% <100 ppb poisoningimpurities N/A: not available; ^(a)calculated based on reportedconversion;

As identified above, Apollo Energy System Inc. (USA) designed an NH₃cracking device to generate H₂ for fuel cell. Hz-containing anode offgas was used as a fuel for combustion to provide thermal energy for thecracker. Conversion to H₂ was high (99.99%) due to the high reactortemperature (480˜660° C.). The energy efficiency was not reported, butit is likely high due to the efficient use of thermal energy from H₂combustion for reactor heating. An efficient commercial catalyst (70 wt% Ni on Al₂O₃) modified with Ru was used. Therefore, a high H₂generation rate was also obtained. Membrane reactors have been studiedfor H₂ generation from NH₃ decomposition.^([9][10]) However, thetechnology is still at the early research stage, and both the H₂generation rate and conversion to H₂ are much lower than ARPA-E'stargets (Table 1).

HYSYS simulation/calculations (Examples 1 and 2) show that the subjectdevelopment is expected to have high H₂ production rate, low H₂delivered cost at target pressure, and high energy efficiency at 400°C., as shown in Table 1. The lower reaction temperature is expected toextend the lifetimes of both membrane and catalyst. NH₃ conversion isexpected to be higher than 99% due to the equilibrium reaction shiftingby selective H₂ extraction. Overall, compared with existingtechnologies, the subject development can or does result in thefollowing advantages:

-   -   Rational and modular design of well-integrated catalyst,        membrane and H₂ burner;    -   High performance of three key components as supported by our        preliminary results;    -   Lower operation temperature (350-450° C.) while        close-to-complete NH₃ decomposition;    -   High H₂ purity (>99%); and    -   Higher energy efficiency and much smaller equipment size        (meaning smaller footprint).

Ammonia, as a promising CNLF and an effective H₂ source, can besynthesized from air and water (N₂ extracted from air and H₂ from water)using renewable energy sources. To produce H₂ as an intermediate, it isessential to develop effective and economic NH₃ decompositiontechnologies. The subject development (such as represented the systemschematic shown in FIG. 1) represents a new and innovative solution. Itsinnovativeness is reflected in the following aspects:

-   -   Low-cost, highly active catalysts for fast NH₃ decomposition;    -   Low-cost highly H₂ selective membrane for extracting H₂ with        purity >99% (simultaneously shifting NH₃ decomposition reaction        towards conversion >99%);    -   Catalytic H₂ burner embedded in the reactor for providing        thermal energy for NH₃ decomposition to achieve overall energy        efficiency as high as 88%; and    -   Membrane in a high packing-density (high surface to volume        ratio) hollow fiber configuration capable of achieving a compact        modular system that can be scaled up linearly by just adding        additional membrane modules.

The present invention is described in further detail in connection withthe following examples which illustrate or simulate various aspectsinvolved in the practice of the invention. It is to be understood thatall changes that come within the spirit of the invention are desired tobe protected and thus the invention is not to be construed as limited bythese examples.

EXAMPLES Example 1 Calculation of H₂ Production Rate in the MembraneReactor

FIG. 5 illustrates an Hz production test system 500 in accordance withone aspect of the subject development. The system 500 includes a NH₃supply source such as an NH₃ cylinder 502, a membrane reactor 504 systemsuch as described above, and a hydrogen product storage, such as ahydrogen product tank 506. The system 500 further include a preheatercross heat exchanger 510 and a cross heat exchanger 512 such as toappropriately preheat the feed (e.g., stream 101) to the membranereactor 502 and to recover heat from the hydrogen product (stream 103)and the reactor flue gas (stream 106), respectively.

The system 500 shows the NH₃ feed stream (stream 101 at 25° C., 10 barand a feed rate of 0.3434 mol/min), permeate stream (stream 103 at 400°C., 2.5 bar, the permeate composed of H₂ at 0.4464 mol/min=10 L/min andN₂ at 0.004509 mol/min), and retentate stream (stream 104 at 400° C.,2.5 bar, the retentate composed of that 0.0687 mol/min and N₂ at 0.1673mol/min) and the membrane reactor 504 operating at 400° C.

H₂ production rate: 10 L H₂/min

Input:

Catalyst:

K-promoted Ru/γ-Al₂O₃

Catalytic activity at 400° C.: 4.5 mmol H₂/min/g catalyst

Packing density: 1 g/cm³

Membrane:

Composite SAPO-34 membrane on hollow fiber (1.5 mm od) housed in metaltube (2 mm od×1.6 mm id)

H₂ permeance at 400° C.: 1.5×10⁴ mol/(m²·s·Pa)

H₂/N₂ selectivity: >700

Catalytic H₂ burner:

Pt-impregnated Ni foam

Heat transfer rate: 1.5 kcal/(cm²·h)

Output:

Catalyst volume: 116 cm³

Membrane area: 0.2 m²

Membrane volume: 133 cm³

Catalytic H₂ burner volume: 10 cm³ (estimated from energy needed fordecomposing NH₃ feed)

Estimated top space of the membrane reactor: 10 cm³

Total membrane reactor volume: 269 cm³

Calculated H₂ generation rate: 0.20 g H₂/h/cm³

Example 2 Energy Efficiency Calculation Using HYSYS Simulation

Referencing the HYSIS simulation system 600 shown in FIG. 6.

HYSYS Flow Diagram:

Stream and Energy Summary:

H₂ production: 10 L (STP)/min and at delivery pressure of 30 bar

${{Energy}\mspace{14mu}{{Efficiency}\left( {E\; E} \right)}} = {\frac{P}{P + E} = {\frac{0.0271 \times 2 \times 33.3}{{0.0271 \times 2 \times 33.3} + {848.7018/3600}} = {88.45\%}}}$

While in the foregoing detailed description this invention has beendescribed in relation to certain preferred embodiments thereof, and manydetails have been set forth for purposes of illustration, it will beapparent to those skilled in the art that the invention is susceptibleto additional embodiments and that certain of the details describedherein can be varied considerably without departing from the basicprinciples of the invention.

What is claimed includes:
 1. A system for generating hydrogen viaammonia decomposition, the system comprising: a fixed bed of a NH₃decomposition catalyst configured to receive inflows of NH₃ andwherewith the NH₃ decomposes to form a combined stream of N₂ and H₂; aplurality of spaced apart and longitudinally aligned ceramic hollowfibers, each of the plurality of spaced apart and longitudinally alignedceramic hollow fibers including an H₂ selective membrane disposedthereon for extracting H₂ from the combined stream of N₂ and H₂ and toform a permeate comprising a high purity H₂ and a retentate comprisingprimarily N₂; and a catalytic H₂ burner extending through the fixed bed,the catalytic H₂ burner comprising a metal tube containing a H₂oxidizing catalyst therein and configured to receive and burn at least aportion of the retentate to provide thermal energy for the NH₃decomposition.
 2. The system of claim 1 wherein the retentateadditionally comprises an amount of H₂ and wherein at least a portion ofthe retentate and an oxidant inflow are introduced into the catalytic H₂burner within the fixed bed reactor to provide thermal energy for theNH₃ decomposition.
 3. The system of claim 1, therein the H₂ burnercomprises a spiral tube configuration extending through the fixed bed.4. The system of claim 3, wherein an outlet end of the H₂ burner metaltube releases N₂ and H₂O.
 5. The system of claim 1 wherein each of theplurality of spaced apart and longitudinally aligned ceramic hollowfibers comprises a porous ceramic material, and additionally comprisinga NH₃ cracking catalyst loaded within the porous ceramic material forcracking of residual NH₃.
 6. The system of claim 5 providing aconversion of ammonia to hydrogen and nitrogen in excess of 99%.
 7. Thesystem of claim 1 wherein the plurality of spaced apart andlongitudinally aligned ceramic hollow fibers are in a parallelalignment.
 8. The system of claim 1, wherein at least a portion of theNH₃ decomposition catalyst is disposed around and in a space betweenadjacent pairs of the plurality of spaced apart and longitudinallyaligned ceramic hollow fibers.
 9. The system of claim 1 wherein amaximum NH₃ decomposition reactor temperature of the fixed bed reactoris no more than 450° C.
 10. The system of claim 1 wherein the inflow ofNH₃ comprises NH₃ vapor at 10-15 bar.
 11. The system of claim 1 whereina conversion of ammonia to hydrogen and nitrogen is in excess of 99%.12. The system of claim 1 additionally comprising a preheater wherebyheat provided by at least one stream selected from the group consistingof a stream of permeated high purity H₂ and a stream of catalytic H₂combustion exhaust preheats the inflow of NH₃ to the fixed bed reactor.